Borides are chemical compounds formed when boron bonds with a less electronegative element. Borides are all hard, high-melting-point materials with metal-like conductivity. They can be made by direct combination of the elements at high temperatures or, more usually, by high-temperature reduction of a mixture of the metal oxide and boron oxide using carbon or aluminium. Chemically, they are stable to non-oxidizing acids but are attacked by strong oxidizing agents and by strong alkalis. Borides can be classified loosely as boron-rich or metal-rich. The generally accepted definition is that if the ratio of boron atoms to metal atoms is 4:1 or more the compound is boron-rich, if it is less, then it is metal-rich.
Boron-rich boride properties vary from one compound to the next, and include examples of compounds that are semiconductors, superconductors, diamagnetic, paramagnetic, ferromagnetic or anti-ferromagnetic. They are mostly stable and refractory. One such compound, lanthanum boride (LaB6), is an inert refractory compound, used in hot cathodes because of its low work function for emission of electrodes.
Borides formed with transition metals tend to form metal-rich borides. Metal-rich borides are typically inert and have high melting temperatures. Investigations into these borides have revealed a wealth of interesting properties such as superconductivity at 39 K in magnesium diboride (MgB2) and the ultra-incompressibility of OsB2 and ReB2.